pelicin › devops › Kubernetes

Labels and Selectors

Labels

Labels are key/value pairs that are attached to objects, e.g., pods. Labels make your cluster more organized, because you can query objects by their labels.

Label keys and values

Label keys are in the form of <prefix>/<name> or <name>, where:

  • prefixes are optional and must be a DNS subdomain
  • prefixes of kubernetes.io/ and k8s.io/ are reserved for Kubernetes core components

Label values must be 63 character or less OR can be empty. Label values may contain alphanumeric characters, dots (.), dashes (-), and underscores (_).

Example: a pod descriptor with labels

For example, a pod description with labels:

apiVersion: v1
kind: Pod
metadata:
  name: label-demo
  labels:
    environment: production
    app: nginx
spec:
  containers:
    - name: nginx
      image: nginx:1.14.2
      ports:
        - containerPort: 80

Selectors

You can use selectors to query for a set of objects.

Types of requirements

The Kubernetes API currently supports two types of selector requirements: (1) equality-based requirement and (2) set-based requirement.

Equality-based requirement

  • equality — e.g. environment = production
  • inequality — tier != frontend

(Note: you can also use == instead of =).

Set-based requirement

  • in — e.g. environment in (production, qa)
  • not in — e.g. environment notin (production, qa)
  • exists — e.g. environment (look for anything that has label key environment, whatever the value)
  • not exists — e.g. !environment (look for anything that DOESN'T HAVE label key environment, whatever the value)

Specifiying multiple requirements

Use comma (,) to make an "AND" clause, e.g.,

tier in (fe, be), environment!=qa

will look for anything whose tier value is fe or be, AND whose environment value is NOT qa.

Working with labels

Showing labels

To show objects with labels:

kubectl get pod --show-labels
# NAME            READY  STATUS   RESTARTS  AGE LABELS
# kubia-manual    1/1    Running  0         16m <none>
# kubia-manual-v2 1/1    Running  0         2m  creat_method=manual,env=prod
# kubia-zxzij     1/1    Running  0         1d  run=kubia

To only show certain labels, use the -L <...keys> flag:

# kubectl get pod -L creation_method,env
# NAME            READY   STATUS    RESTARTS   AGE   CREATION_METHOD   ENV
# kubia-manual    1/1     Running   0          16m   <none>            <none>
# kubia-manual-v2 1/1     Running   0          2m    manual            prod
# kubia-zxzij     1/1     Running   0          1d    <none>            <none>

Querying with labels

Use the kubectl get command with -l <selector>, e.g.:

# Enclose `!<something>` in single quotes
# so bash doesn't evaluate the '!'
kubectl get pod -l creation_method=manual
kubectl get pod -l '!env'

Modifying labels

Use the kubectl label command.

kubectl label pod kubia-manual creation_method=manual
# pod "kubia-manual" labeled

Use the --override flag to override an existing label.

kubectl label pod kubia-manual-v2 env=debug --overwrite
# pod "kubia-manual-v2" labeled

References